Power Amplifier Efficinecy

Hi,

I designed a Power Amplifier which I drew its schematic below.
View attachment 67036

The Input Power is constant, and I control the Power Gain of the PA by changing the values of IREF1 and IREF2 (controllable Current Sources).

When I decrease IREF1 & I REF2, the Power Gain decreases as well (which is what I wanted to achieve), but also the Efficiency of the Power Amplifier decreases.

When I decrease VDD (shown in Schematic) in parallel to decreasing IREF1 & IREF2, the Efficiency drops less drastically.

Please see Efficiency Graph below with Constant VDD Vs. Regulated VDD - as function of the Power Gain.
View attachment 67037

The first thing I'm trying to achieve is understanding why the Efficiency decreases as IREF1 & IREF2 decrease, with Constant VDD.

Can you please share some wisdom and guidelines regarding the Efficiency decrease?

Thank you very much.

What are MN and ISM?
Are power losses in MN or ISM affected by IREF1 or IREF2?
How do you define Efficiency?

alec_t said:

What are MN and ISM?
Are power losses in MN or ISM affected by IREF1 or IREF2?
How do you define Efficiency?

Click to expand...

Hi and thanks for your comments - please find answers below.

1.
MN is an ideal Matching Network - one to match input source (RF_IN) with PA and one to match PA with Load.
ISM is an ideal Inter-Stage Matching Network that matches the two stages.

2.
I assume no power losses in the Matching Networks (MN & ISM) - They are considered to be ideal.

3.
I define Efficiency as Drain Efficiency = AC Output Power / DC Power
Where:
AC Output Power = RMS{AC Output Current * AC Output Voltage} (i.e. the current and voltage delivered to the Load; No DC Signal is transferred to the load).
DC Power = VDD * (Idrain) [ Idrain = ICE(BJT1) + ICE(BJT2) ]

The first thing I'm trying to achieve is understanding why the Efficiency decreases as IREF1 & IREF2 decrease, with Constant VDD.

Click to expand...

A simulation in LTSpice confirms efficiency decreases with decreasing bias current. IMO this is due to the resistance of the base-emitter junctions decreasing as the bias current increases. Reduced resistance leads to increased RF current from the source.